1. Field of the Invention
The present invention relates to a process for producing copies in which a transferable image is formed on an electrophotographic element having a photoconductive layer. The transferable image is formed on the electrophotographic element by charging, exposing image-wise, and developing with a developing powder. Then, the image is transferred onto copy material whereupon it is fixed. More specifically, the invention relates to an electrophotographic element used in the above process comprising a photoconductive pigment dispersed in a binder consisting substantially of a copolymer of an acrylic or methacrylic acid ester monomer, a vinyl aryl monomer and 1% to 3% by weight unsaturated acid.
2. Description of the Prior Art
A method of producing copies using the above-described process and electrophotographic element has been disclosed in the prior art. For example, a process of this kind is described in United Kingdom Pat. No. 1,510,200 and its corresponding Dutch Patent Application No. 7,611,130. The electrophotographic element described in this United Kingdom Patent has a photoconductive layer comprising zinc oxide dispersed in a polymeric binder such as the copolymer of styrene ethyl acrylate. The preferred styrene ethyl acrylate copolymer for electrophotographic use is marketed under the name Synolac 620S and contains a copolymer of styrene, ethyl acrylate, and acrylic acid in the ratio of 1.5:1.0:0.025.
The use of zinc oxide dispersed in a polymeric binder as the photoconductive layer in an electrophotographic element is known in the prior art. U.S. Pat. No. 3,929,480, teaches the use of acrylic or alkyd polymer resins or a mixture of both as the polymeric binder in a photoconductive layer. This patent, however, teaches the use of specific polymer resins such poly-.alpha.-methyl styrene as an additive to improve the optical qualities and does not include an unsaturated acid such as acrylic acid. Similarly, Japanese Patent Abstract No. 57-202544 describes a polymeric binder consisting of 2-hydroxy ethyl acrylate, acrylic acid, styrene, and (meth)acrylic ester. It is evident from the Japanese abstract that the ratio of styrene to acrylic or methacrylic acid ester monomer is about 1.5 to 1.0, the same as in Synolac 620S.
When an electrophotographic element containing Synolac of the type described in United Kingdom Pat. No. 1,510,200 is used in a process in which images of different sizes are formed successively on the electrophotographic element and then transferred onto copying material with heat, aging of the electrophotographic element becomes visible. Typically, a contour of a smaller image becomes visible on a larger image that is formed on the same place of the electrophotographic element after the smaller image. The various circumstances surrounding this phenomenon, which hereafter will be referred to as format outlining, are only partially known. The phenomenon is visible on large size copies and usually occurs after 1,000 to 10,000 copies have been made and is caused by the fact that the parts of the photoconductive layer on which images of large and small sizes coincide are used more frequently than the parts on which only an image is formed when large copies are made. The phenomenon is not visible when copies of only one size, either large or small, are made exclusively.